1. Field of the Invention
The present invention relates to a modulation/light sensing device capable of high-density and large-volume recording and reproduction of information by recording interference fringes of light on a recording medium, which device is used in an optical information recording and reproducing apparatus using a hologram.
2. Description of the Related Art
The world is in a multimedia age today, and there has been an increasingly greater necessity for a recording apparatus for performing recording on a recording medium and a recording and reproducing apparatus for performing recording of information on and reproduction of information from the recording medium, and the recording density thereof is also rising year after year. As for optical information recording media, progress has been made from a CD (Compact Disc) to a DVD (Digital Versatile Disc) and then to a Blue-ray disc. In response to the progress in the optical information recording media, the recording density is also increased by using shorter-wavelength light as to an optical information recording apparatus and an optical information recording and reproducing apparatus. In recent years, a new recording method, called a holographic memory, has been proposed. The holographic memory records information by forming holograms according to the information to be recorded in the recording medium. It is capable of multiple recording due to a characteristic of using the holograms, in which the information is reproducible independently from the holograms, even if adjacent holograms have mutually overlapping portions. Therefore, the holographic memory can attain a high recording density that is not obtainable by a conventional optical information recording medium.
The holographic memory is described, for example, in “Holographic Media Close to Takeoff, Realize 200 GB in 2006” by Hideyoshi Horigome, et al., pages 105 to 114, Nikkei Electronics, No. 891, Jan. 17, 2005. This article discloses a recording and reproducing apparatus based on the holographic memory using a two-dimensional spatial light modulator and a light sensing device. This article also describes an optical system of a coaxial type holographic memory system (optical information recording and reproducing apparatus) called a collinear system.
After all, the optical information recording and reproducing apparatus develops the information to be recorded as two-dimensional digital pattern information, and an information light beam is modulated by the two-dimensional digital pattern information. This process generates an information light beam wherein recording information has become light intensity distribution image information of two-dimensional space. And, it lets the information light beam and a reference light beam interfere so as to record interference fringes thereof on the recording medium. With regard to reproduction, it extracts and decodes the two-dimensional digital pattern information from the light intensity distribution image information reproduced by irradiating the reference light beam on the recording medium. It is possible, by this digital process, to suppress lowering of a reproduction error rate due to degradation of an S/N ratio, and to correct an error by encoding binary data, so as to reproduce the recording information extremely faithfully.
The above-mentioned collinear holographic memory system has a coaxial optical arrangement in which the information light beam and the reference light beam have no angle, so that recording and reproduction can be performed by using one objective. For that reason, there is an advantage that the optical system becomes simple, in comparison with a biaxial two-beam interference method of irradiating the information light beam and the reference light beam on the recording medium from different optical paths. There is also an advantage that the optical system can be placed on one surface side of a disk-like recording medium, because of a structure of the recording medium having a reflective film.